The generalization of the use of the IP (“Internet Protocol”) protocol in telecommunications networks has allowed the implementation of new services, whether for the Internet, telephony or for audiovisual. Such services are proposed, on the basis of dedicated architectures, on the fixed network and on the mobile network.
It is known to adapt the distribution of multimedia content to the characteristics of the user terminal and of the access network within one and the same access network, be it fixed or mobile. In particular, the person skilled in the art knows how to dynamically alter the distribution of a multimedia content, when the user terminal changes type of mobile cell and passes, for example, from an HSDPA (“High Speed Downlink Packet Access”) cell to an EDGE (“Enhanced Data rates for GSM Evolution”) cell, the EDGE cell offering a lower throughput than the HSDPA cell, typically 50 kbps instead of 250 kbps. Accordingly, it implements a mechanism based on an analysis of RTCP (“Real-Time Transport Control Protocol”) reports transmitted by the user terminal to the contents server by RTP (“Real-Time Transport Protocol”) return pathway. In the case where a succession of erroneous reports is received, the server triggers the distribution of the same content with a lower quality level. It is nevertheless noted that this adaptation is done within one and the same mobile network.
The proliferation of dedicated IP-based services and architectures poses the problem of fixed network—mobile network convergence. In this regard, the standardization body for 3GPP (“3rd Generation Partnership Project”) mobile networks is currently proposing two technical solutions allowing the convergence of fixed and mobile architectures:                the first solution, based on the GAN (“Generic Access Network”) protocol, supports the mobility of a user terminal between a fixed network and a mobile network of GSM (“Global Systems for Mobile Communications”) type for conversational services and GPRS (“General Packet Radio Service”) type for services related to packet mode data transport. A telecommunications network 1 comprising a first mobile access network 31, a second fixed access network 32 and a services platform 60 comprising a contents server 61 is presented with reference to FIG. 1. A user terminal 10, attached to the mobile access network 31 by way of an access point, of the base station type 21, is considered. This terminal is currently displaying an audiovisual content transmitted in the course of an audiovisual session that it has established with the contents server 61. The constituent data stream of the encoded audiovisual content is transmitted to it by way of the mobile access network 31 and, more precisely, via an input gateway to the mobile access network of GGSN (“Gateway GPRS Support Node”) type, an SGSN (“Serving GPRS Support Node”) item of equipment in charge of verifying a user's rights to the requested service and of allocating end-to-end the resources required for the implementation of this service, and an RNC (“Radio Network Controller”) controller in charge of controlling the mobility of the user terminal from one base station to another and of verifying that the resources required for the implementation of the service are available on the link between the RNC, the base station and the user terminal. When the user terminal enters the zone of coverage of the fixed access network 32, it attaches itself to a radio access point linked to a domestic gateway 22 to the fixed access network 32. According to the prior art, the switchover of the audiovisual session is carried out by a controller GAN-C, situated in the mobile access network, downstream of the GGSN and which has means for carrying the data stream toward the fixed access network. It relies on an item of control equipment, called the GAN-C, which comprises means for switching over a data stream from a first interface with the mobile network to a second interface to the fixed network. A drawback of this technical solution is that the item of control equipment GAN-C is connected to the SGSN by a link which has a limited bandwidth to ensure real-time packet mode data streaming. In particular, this solution is the best for allowing the user terminal to continue to receive, by way of the fixed access network, the audiovisual content that it was currently receiving on the mobile access network, but in no case does it allow it to profit from the widened bandwidth offered by the fixed access network. Another drawback of this technique is that it monopolizes part of the resources of the mobile network to transmit a data stream to the fixed access network (from the GGSN up to the GANC);        the second technical solution, called I-WLAN (3GPP-Wireless Local Area Network Interworking), is more suited to data transport. It allows a mobile operator to offer its subscribers a service for data transport from a fixed access network. Today, it has means for ensuring the mobility of conversational services from the GSM network to a packet mode fixed network, with the aid of the Voice Call Continuity (VCC) specification. Nevertheless, today it offers no solution for the mobility of audiovisual contents distribution services.        